Abstract

Although gibberellin (GA) has been reported to control branching, little is known about how GA mediates signals regulating the outgrowth of axillary buds (ABs). In the current study, the effect of the exogenous application of 5.0 mM GA3 on ABs outgrowth on 1-year-old ‘Nagafu No. 2’/T337/M. robusta Rehd. apple trees was investigated and compared to the bud-activating treatments, 5 mM BA or decapitation. Additionally, the expression of genes related to bud-regulating signals and sucrose levels in ABs was examined. Results indicated that GA3 did not promote ABs’ outgrowth, nor down-regulate the expression of branching repressors [MdTCP40, MdTCP33, and MdTCP16 (homologs of BRANCHED1 and BRC2)], which were significantly inhibited by the BA and decapitation treatments. MdSBP12 and MdSBP18, the putative transcriptional activators of these genes, which are expressed at lower levels in BA-treated and decapitated buds, were up-regulated in the GA3 treatment in comparison to the BA treatment. Additionally, GA3 did not up-regulate the expression of CK response- and auxin transport-related genes, which were immediately induced by the BA treatment. In addition, GA3 also up-regulated the expression of several Tre6P biosynthesis genes and reduced sucrose levels in ABs. Sucrose levels, however, were still higher than what was observed in BA-treated buds, indicating that sucrose may not be limiting in GA3-controlled AB outgrowth. Although GA3 promoted cell division, it was not sufficient to induce AB outgrowth. Conclusively, some branching-inhibiting genes and bud-regulating hormones are associated with the inability of GA3 to activate AB outgrowth.

Notes

Funding

This work was supported by the National Apple Industry Technology System of Agriculture Ministry of China (CARS-28); Yangling Subsidiary Center Project of National Apple Improvement Center and Collaborative Innovation of Center Shaanxi Fruit Industry Development (C000088); Chinese postdoctoral project (2015M582713); Innovation project of science and technology plan projects of Shaanxi province (2016TZC-N-11-6).

Compliance with ethical standards

Conflict of interest

All authors declare that they have no conflict of interest. Ming Tan declares that she has no conflict of interest. Guofang Li declares that he has no conflict of interest. Xiaojie Liu declares that he has no conflict of interest. Fang Cheng declares that she has no conflict of interest. Juanjuan Ma declares that she has no conflict of interest. Caiping Zhao declares that she has no conflict of interest. Dong Zhang declares that he has no conflict of interest. Mingyu Han declares that he has no conflict of interest.

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